Improve antialiased line drawing sharpness and respect of original width

Antialiased lines with a width of 2 pixels or more now use a smaller
feather width of 1.25 instead of 2.0. This makes them look sharper
and better preserves their original shape while still having good
antialiasing quality.

This also improves the appearance of thin antialiased lines by
slightly increasing their feather size (from 1.0 to 1.25). It
makes them appear a tad thicker, but the antialiasing quality is
much improved by doing this.

servers/rendering/renderer_canvas_cull.cpp

@@ -594,9 +594,13 @@ void RendererCanvasCull::canvas_item_add_line(RID p_item, const Point2 &p_from,
 	}
 
 	if (p_antialiased) {
-		float border_size = 2.0;
-		if (p_width < border_size) {
-			border_size = p_width;
+		// Use the same antialiasing feather size as StyleBoxFlat's default
+		// (but doubled, as it's specified for both sides here).
+		// This value is empirically determined to provide good antialiasing quality
+		// while not making lines appear too soft.
+		float border_size = 1.25f;
+		if (p_width < 1.0f) {
+			border_size *= p_width;
 		}
 		Vector2 dir2 = diff.normalized();
 
@@ -763,9 +767,13 @@ void RendererCanvasCull::canvas_item_add_polyline(RID p_item, const Vector<Point
 	Color *colors_ptr = colors.ptrw();
 
 	if (p_antialiased) {
-		float border_size = 2.0;
-		if (p_width < border_size) {
-			border_size = p_width;
+		// Use the same antialiasing feather size as StyleBoxFlat's default
+		// (but doubled, as it's specified for both sides here).
+		// This value is empirically determined to provide good antialiasing quality
+		// while not making lines appear too soft.
+		float border_size = 1.25f;
+		if (p_width < 1.0f) {
+			border_size *= p_width;
 		}
 		Color color2 = Color(1, 1, 1, 0);